It is estimated that when the weight of a vehicle is decreased by 10%, its fuel consumption will be lowered by 5%-8%, and the emission of greenhouse gas CO2 and such pollutants as NOx, SO2, etc., will be reduced as well. Self-owned brand passenger vehicles of our country are approximately 10% heavier than their foreign counterparts, and the difference in weight is even larger for commercial vehicles. Automobile steel plate, which is the main raw material of an automobile body, accounts for about 60-70% of the weight of the automobile body. Mass use of high-strength and super-high-strength steel plate with strength at the level of 590˜1500 MPa instead of traditional automobile steel is an optimal solution to the problem of material in order to achieve “reduced weight, less energy consumption, higher safety and lower manufacturing cost” for automobiles, and it is also of great significance for the building of low-carbon society. Hence, it has been a trend in recent years for the development of steel plate to enhance the strength of the steel plate so that the thickness of the steel plate can be reduced. Development and application of advanced high-strength automobile steel mainly strengthened by phase change has been one of the mainstream subjects under research in various large steel companies in the world.
The high strength of traditional super-high-strength steel is originated from the high-strength phase structure of martensite, bainite, etc., but the plasticity and the formability are reduced significantly at the same time. Introduction of a certain amount of residual austenite into the structure of martensite or bainite is an effective technical approach to obtain materials with high-strength and high-plasticity. For example, TRIP steel is composed of ferrite, bainite and residual austenite, and has relatively high strength and plasticity, but this phase structure restricts the further improvement of its strength. Thus, replacement of bainite by martensite as the main strengthening phase has begun to gain attention.
Chinese Patent CN 102409235A discloses a high-strength cold-rolled transformation-induced plasticity steel plate and preparation method thereof, wherein the composition is: C: 0.1%-0.5%, Si: 0.1%-0.6%, Mn: 0.5%-2.5%, P: 0.02%-0.12%, S≤0.02%, Al: 0.02%-0.5%, N≤0.01%, Ni: 0.4%-0.6%, Cu: 0.1%-1.0%, and the balance of Fe. The preparation method comprises the following steps: (a) smelting molten steel meeting the composition condition, and casting into a blank; (b) rolling, wherein the heating temperature is 1100-1250° C., the heat preservation time is 1-4h, the initial rolling temperature is 1100° C., the end rolling temperature is 750-900° C., the coiling temperature is lower than 700° C., the thickness of a hot-rolled steel plate is 2-4 mm, and the cold-rolling accumulated reduction amount is 40-80%; and (c) continuous annealing, wherein the annealing temperature is 700-Ac3+50° C., the heat preservation time is 30-360 s, the cooling speed is 10-150° C./s, the aging temperature is 250-600° C., the aging time is 30-1200 s, and the steel plate is cooled to room temperature at a speed of 5-100° C./s. The steel plate of the invention has a yield strength of 380-1000 MPa, a tensile strength of 680-1280 MPa and an elongation of 15-30%. An elongation of about 20% can be realized by the invention on a tensile strength level of 1000 MPa, and the steel plate has relatively good comprehensive properties. However, a relatively large amount of alloy elements such as Cu, Ni and the like are added into the steel of the invention, which increases the material cost to a large extent, and notably restricts its application in the automobile field which has extremely critical demand on cost.
Japanese Patent JP 2005-232493 discloses the composition of a steel plate having high strength and high formability as well as a process. The composition comprises C: 0.02-0.25%, Si: 0.02-4.0%, Mn: 0.15-3.5%, and the balance of Fe. The structure of the material comprises double phases of ferrite and martensite, wherein the ferrite content accounts for 30-60%. The content of residual austenite is less than 1.0%. The coiling temperature of the hot-rolled plate is 500° C., and the plate is heated to 900-950° C. after cold rolling, followed by slow cooling to 640° C., then quick cooling to 350° C., and finally slow cooling to room temperature. Steel plate having about 850 MPa of yield strength, about 1000 MPa of tensile strength and 14% of elongation can be obtained via the above process. The steel of this invention features simple composition and low cost, but the elongation on the order of 14% still can not satisfy the demand of automobile high-strength steel on formability.
Chinese Patent CN200510023375.0 discloses a low-carbon, low-silicon cold-rolled transformation plasticity steel and a manufacturing method thereof. The components and weight percentages of the low-carbon, low-silicon cold-rolled transformation plasticity steel of this invention are: C 0.1-0.2%, Si 0.1-0.5%, Mn 0.5-2.0%, Al 0.5-1.5%, V 0.05-0.5%, trace amount of S, P, N, and the balance of Fe. After treatment, the low-carbon, low-silicon cold-rolled transformation plasticity steel exhibits good strong plasticity, 650-670 MPa of tensile strength and 32.5-34% of elongation. The steel of this invention has low tensile strength, and thus can not meet the demand of automobile super-high-strength steel on performance properties. Moreover, addition of a certain amount of Cr is required, rendering it unsuitable for use as automobile steel which has very critical demand on cost control.